Many mods simply change or add another keyword to the item (ex. adding keyword materialweaponsteel to draugr axe so steelsmithing will pick it up) to enable the double improvement. Where in the creation kit can I add other armor/weapon keywords to the PERK-like how advanced armorsmithing has scaled and plate armor?

How about.... in the perk? Seriously, it's that easy. Perks are described in the "Perks" category in the object window, within the top level "Actors" category. Find a perk, like "Advanced Armors", double-click it. Double-click the "Modify tempering health" item in "Perk entries", and click over to the "Item" tab in the condition window. The conditions entered there determine which items will be affected by the perk.

Note that this only affects whether items can be upgraded, not whether they can be created at all. Controlling which items can be created when you have a perk takes place in the "Items/Constructible Objects" category. Recipes have conditions specifying whether they show up at the crafting work spaces.

It's all a little buried, but once you get a grasp of the keyword and conditions concepts, it's not that hard to figure out where to go looking for the way to do something.